mmc: mxs-mmc: convert to clk_prepare/clk_unprepare

The patch converts mxs-mmc driver to clk_prepare/clk_unprepare by
using helper functions clk_prepare_enable/clk_disable_unprepare.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Chris Ball <cjb@laptop.org>
Acked-by: Marek Vasut <marek.vasut@gmail.com>